Two routes for cobalt doping in ZnO nanostructures

P. Hari and A. Kaphle
University of Tulsa,
United States

Keywords: ZnO, Nanomaterials, Doping, Cobalt Doping

Summary:

Two routes for cobalt doping in ZnO nanostructures Controlled growth of ZnO nanorods on various substrates is of great importance in photonic and electronic devices. Also of interest is increasing the optical activity of zinc oxide nanorods in the visible spectrum. In this study, we report doping dependence on the morphology, optical, absorption, electrical conductivity and photoluminescence (PL) of aligned ZnO nanorods doped with cobalt from 5 % to 20% grown on glass substrates. The ZnO nanorods were grown by a chemical bath deposition (CBD) technique using equimolar ratios (selected at 0. 1M and 1 M) of zinc (II) nitrate and hexamethylenetetramine in solution at 95◦C. Doping is achieved by adding cobalt nitrate to the precursor solutions of the CBD in one set of samples and by adding cobalt chloride to the CBD precursor solutions in a second set of samples. We compared the doping efficiency in two sets of samples doped with cobalt via cobalt nitrate and cobalt chloride. Measurements of the cobalt incorporated in ZnO matrix were performed using atomic emission spectroscopy (AES). AES measurements show that cobalt incorporation is much more efficient at 1M concentration of the precursor solution using cobalt chloride than using cobalt nitrates for doping. Scanning Electron microscopy (SEM) images show that at 0.1 M ratio of the CBD precursor solutions, the morphology of ZnO deposited resulted in hexagonally shaped nanorods. The nanostructure morphology is maintained in 0.1 M cobalt doping, irrespective of the route (cobalt nitrate or cobalt chloride) taken to achieve the doping. At 1M ratio of the precursor solutions, SEM images show that the morphology revealed platelets at all doping levels, irrespective of the doping method used. Electrical conductivity measurements at 300K using Van der Pauw method show that ZnO nanorods doped with cobalt chloride yielded the lowest resistivity at 0.1 and 1 M. In addition, PL spectra of the doped samples show a peak shift to longer wavelengths in both cobalt nitrate and cobalt chloride samples. The near band edge in absorption measurements show a progressive shift in band gap to lower energies in both types of doping. However, the band edge shift was more pronounced in 0.1 M cobalt nitrate samples compared to other samples. Based on the cobalt incorporation efficiency, doping of ZnO by cobalt chloride may be a better method for achieving higher doping efficiency by chemical bath deposition.